1. Field of the Invention
The present invention relate to a method and apparatus for the in situ addition calibration of an inductively coupled plasma atomic emission spectrometer or mass spectrometer using a precision gas metering valve to introduce a volatile calibration gas of an element of interest directly into an aerosol particle stream, and in particular, use of the present in situ calibration technique with various remote, on-site sampling systems.
2. Description of the Related Art
Inductively coupled plasma atomic emission spectrometers and mass spectrometers can be calibrated by adding a series of standardized additions to the sample being tested. A standard addition curve for the analyte being measured can then be generated. However, in some circumstances it is difficult or not possible to add the standards to the sample.
For example, there is an ongoing need to sample and analyze dangerous or hazardous materials, or materials located in hazardous environments such as soil or water at hazardous waste sites (radioactive wastes, toxic chemical dumps or contaminated structures) or molten metals in a manufacturing foundry. Conventionally, a sample of a hazardous waste is removed from the site and brought to a laboratory for analysis. The sample must therefore be carefully extracted, transported, handled and stored in order to assure the safety of the technicians carrying out the test, as well as the public. The expense and delay entailed in extracting, handling and storing such materials, as well as the health risks, have encouraged scientists to develop alternative testing approaches minimizing these disadvantages.
The Iowa State University Research Foundation has developed a system for analyzing the composition of specimens directly at a sample site. This system is disclosed in a U.S. patent application Ser. No. 08/117,242, entitled MOBILE INDUCTIVELY COUPLED PLASMA SYSTEM, filed by A. D'Silva and E. Jaselskis, and further disclosed in published PCT application No. WO 93/07453, both of which are hereby incorporated by reference. However, the amount of a sample delivered to the inductively coupled plasma (ICP) or mass spectrometer varies with laser output power and power density at the sample surface, light scattering from aerosol particles in the ablation cell, and variations in aerosol transport out of the cell and through the transfer tubing to the ICP torch.
In order to quantitate and normalize on-site samples, a method and apparatus for determining in situ the mass and concentration of elements of interest is needed for use with inductively coupled plasma atomic emission spectrometers (LA-ICP-AES) and a mass spectrometers. Various methods for normalization and quantitation in laser ablation sampling are described in D. Baldwin, D. Zamzow, and A. D'Silva, "Aerosol Mass Measurement and Solution Standard Additions for Quantitation in Laser Ablation-Inductively Coupled Plasma Atomic Emission Spectrometry", 55 Anal. Chem. 1911, 1917 (1994), which is hereby incorporated by reference.
The method disclosed in the above noted article combines the technique of aerosol mass measurement and solution standard additions. A portion of the laser-ablated sample aerosol is diverted to a quartz microbalance and the mass flow rate is measured. During the laser ablation sampling process, a measured amount of a desolvated aerosol obtained from ultrasonic nebulization of solution standards is added to the laser-ablation aerosol to generate a standard addition curve for the analyte being determined.
However, ultrasonic nebulizers are not 100% efficient. Failure to nebulize all of the liquid in the standard will introduce error into the system. Nebulization may also be effected by temperature and pressure. Additionally, the liquid standards utilized with this technique typically contain dangerous concentrations of acids and can be unstable over time. Finally, the overall complexity of the pumps and valves necessary to introduce the nebulized standard into the ablation stream are not well suited to automation and may impact on the reliability of the system.